No sure what GRP is, but assuming it is the fiberglass gel coat on your boat.
Never easy but I use a low speed 7" angle grinder/polisher and wool bonet, with 3M liquid polishing compound.
Follow up with a buffer and you favorite brand of wax-like new!

Heck if it was easy all of our boats would look brand new forever and you wouldn't be able to get a great deal on a used boat that need a little TLC!

The following is information pertaining to the general process for marine detailing. Due to the vast differences in ages of the vessels and differences in gelcoat types and conditions of said vessel, we can not be correct on all issues.

Use this page as a brief overview for the proper procedure for marine detailing only, ask for the proper product types to use on your vessel before attempting.

OVERVIEW

Hand applications can achieve satisfactory results.
We prefer the use of a power buffer (non orbital types) for various reasons.
- Applies a thinner coat, making product easier to remove.
- Products coat evenly, resulting in a uniform finish.
- Clean more aggressivly, removes more contaminants and
oxidation.
- When used properly it will remove swirl marks
(can cause swirl marks if wrong or dirty wool pads and products are used.)
It is up to the consumer to varify the level of oxidation/current state of vessel.

STEP 1 COMPOUND (Heavy)

Compounds vary in degrees of grit/courseness. They are used to remove heavy oxidations and deeply imbedded contaminants.
Various compounds can either be in a paste form (brushed on, heavier grit) or in liquid form (milder grits).
When using compounds, it is best to do small areas at a time, 4' x 4' is the recommended size. When using compounds, use 100% WHITE wool pad.
Apply product to hull. If using paste compound, brush evenly in a 4' x 4' area. If liquid form, apply in vertical stripes spaced 9" - 11" apart, top to bottom of area. Check product recomendation for proper RPM of the power buffer, usually between 1500-2000 RPM. While using buffer, it is important to keep it moving. Overlap your strokes by about 50% while using light pressure. When using compound it isbest to leave the buffing pad flat on the surface to minimize swirl marks. While working product in, areas you have gone over in the section will be drying. Once the section is worked in, spur the pad. (spurring details below). Go back over section and remove remaining compound. When moving to
the next section overlap previous section by 10". Continue until hull/topside is complete.

STEP 2 COMPOUND (Mild)

Mild compounding is the same process except you will be dealing with a liquid product of finer grit. This is used to remove mild oxidation. 100% WHITE wool pad is still used in the mild compound stage.

STEP3 POLISHING

Polishes are used to remove slight oxidation and remove fine scratches left by the compounding stages. It is also used to remove swirl marks. Swirl marks are microscopic scratches in the gelcoat caused by many reasons i.e. washing with stiff brushes, compounding, applying or removing products with dirty or improper or otherwise harsh applicator.
Swirl marks are identifiable in direct sunlight and appear ,as swirls of course, on the surface. The most noticable region on a boat for swirl marks in the the forward sections in the bow where the flair is and the front of the flying bridge topsides.
Polishes should be applied in 4' x 4' sections also, but on newer boats of certain manufactuers can be applied in larger sections. Apply the polish in vertical lines spaced 12" apart.
Using a power buffer with 100% YELLOW wool pad, apply the product, working it completly in. It is best to overlap your sections and work slow. Polish restores beneficial oils to the gel coat which can prolong it's shine. As you finish working the product in, spur the pad and remove from surface.

STEP 4 WAXING

Waxing is one of the easiest ways to extend the amount of time before you need to utilize steps 1-3. There area many different variations of ways on the market. Some that are even a polish, or vise versa. The highest priced and highest quality is YELLOW Carnuba. Polymer and synthetic polymer coatings are also good, but are limited to the types of gelcoat/paint they can be used on. Teflon waxes are not what they claim to be either. DuPont, the maker of Teflon, issued this statement:
"The application of a Telfon Fluoropolymer resin does nothing to enhance the properties of a car wax. We have no data that indicates the use of Teflon is benificial in car waxes."
Wax is best applied with the appropriate wax applicator applied in circular motions. It is best to do small sections, even as small as 2' x 2' to avoid "baking in" We do not recommend applications of 2 or more coats at a time. It is better to apply wax at more
frequent intervals to prevent oxidation. Wax the vessel every one and one half month to two months. More so in hotter, troplical regions and also salt water locations. When removing wax, always use a 100% terry cloth towel that is made in the U.S.A.

SPURRING/CARE/USEAGE OF WOOL PADS
1) Spur the pad using the spur usually supplied with pad. (Tounge depressors work great for this.)

Doesn't get much more complete then that!
I have used most products on the market over the years - but I keep comming back to 3M. Not the cheapest but nothing that works ever is :wink:
Have used Zanio on my cars with great results!

My personal favorite is the Blue liquid fiberglass cleaner produced by Surebrite. They also make a white liquid, however it does not work as well. The blue stuff comes in a blue bottle. It removes wax and scratches and stains with very little effort. I am amazed at the results achieved when it comes to scratches.

The drawback is that you will need to apply a good wax to the surface once it has been cleaned. I have been using Maguires yellow carnuba wax which give a good shine, but is manually intensive. Although I have an electric buffer, I have not been able to master its use. It seems to be more work than just hand buffing with a terry cloth. The basic problem I see is that the pad fills with residue and unlike a cloth you can shake out, there is no easy way to unclog the buffer pad. The other difficulty is the actual bottom. I am only waxing the 1st half of the boat as the axle's get in the way. Would love to hear some tips on how to make it easier.